Hirschsprung disease (HSCR) is a congenital disorder of the gastrointestinal tract that presents as a lack of ganglia in a variable portion of the distal colon due to deficits in the neural crest stem cells (NCSC) that give rise to the enteric nervous system. HSCR is a prime example of a complex genetic disease. Often HSCR cases are attributed to mutations in RET but mutations in other genes, including SOX10, can also cause aganglionosis. Approximately 50% of HSCR cases are of unknown etiology. Extent of aganglionosis varies widely among individuals and can even differ between family members. Studies in mouse models of HSCR demonstrate that modifier genes in the genetic background play a large role in determining both the penetrance and severity of aganglionosis. Understanding modifiers and gene-gene interactions that contribute to variation of HSCR will increase understanding of HSCR complexity and pathogenesis. Previous studies have established the utility of Sox10Dom mice in the study of HSCR. These mice recapitulate the aganglionosis seen in patients and have been used to identify modifiers of aganglionosis. The current study aims to demonstrate the interaction of a highly significant candidate gene with Sox10 to impact the aganglionic phenotype of the Sox10Dom HSCR model and will definite the biological effect of this interaction on enteric NCSC. Aim 1 will use BAG modification techniques and the generation of transgenic lines of mice to demonstrate that altered levels of the identified candidate gene modify the penetrance and severity of Sox10Dom aganglionosis. Aim 2 will use both immunohistochemistry and culture of purified NCSC to determine the effect of the modifying interaction on migration, proliferation, and developmental potential on NCSC. The results of this study will provide the gastroenterology community with a greater understanding of the causes of HCSR as well as the biological processes involved in the development of the enteric nervous system. Finally, characterization of the Sox10-modifier interaction will provide insight into how modifiers influence other complex diseases. This research will increase our understanding of the genetic basis of HSCR, help identify causes of aganglionosis in patients with non-classifiable disease, and increase knowledge of enteric nervous system development.